Amplifier



Feb. 20, 1940 J. o. EDSON ET AL- AMPLIFIER 2 Sheefs-Shee't 1 Filed May 27, 1936 J. 0. .5050 a. m; so/v INVENTOR$= n m T T A Feb. 20, 1940.

J. o. EDSON ET AL 2,191,167

AMPLIFIER Filed May 27, 1936 v 2 Sheets-Sheef 2 GAIN CON TROL FIG/3 .J.O. EDSON lNl/ENTORS a. MASON A TTORNEV Patented Feb. 20, 1940 UNITED STATES rrics PATENT 2,191,167 AMPLIFIER James O. Edson, Great Kills, and Ira G. Wilson, Brooklyn, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 27, 1936, Serial No. 82,156

' 4 Claims. (Cl. lie -171) ities in the drain and without undue coupling or ,,,,cioss-talk between amplifiers supplied by the same battery.

Further objects and advantages of this invention will appear from consideration of the specificationand drawings illustrative of typical embodiments of the invention in which drawings:

fortfilamentlighting and for plate current, the full voltage of which is employed for plate voltage and successive sections of which are tapped for filament lighting for separate repeater amplifiers;

Fig. 2 shows the adaptation of the usual power batteries at an existingrepeater station to the amplifying system of Fig. 1; and l Fig. 3 shows a possible adaptationof an existwing amplifying system at a repeater station to employ the filament'and plate battery in series for plate voltage on all tubes with provision against undue cross-talk between the amplifiers connectedto the common batteries/ .Fig; l showsschematically a group of amplifiers A, B and C to N, each amplifier being associated with a communication circuit or telephone pair, of which a large number are employed, for example, in a leadc'overed cable in a -carrie1" telephone-system. The circuits may be open wire pairs, radio circuits or anyusuitable medium as well as. those of a lead covered telephone cable or the like. The amplifier-A is associated with one of the circuits or pairs 9 through an input transformer it having suitable shielding. The secondary winding er the transformer H1 is connected in this case to the control grid of a power pentode ll of the type knownas 3l0-A Western Electric pentode tube. The gain of the amplifier is controlled at l2 by some suit able means not shown in detail for varying imp'dance in shunt of the circuit connecting the input to the output bridge i5. The tube itself is self-biased by resistance [30in shunt with a condenser M.

Fig. 1 .shows a single power source, used both The amplifier A of itself forms no part of the present invention and will not be described in all its details. ,It is chosen for illustration because it represents a type of repeater to which the c power supply circuits of the invention are well ,adapted. For information as to structure and performance of a repeater of generally similar type, reference may be made to U. S. patent to H. S. Black No. 1,956,547 granted May 1, 1934.

The tube H is connected to a succeeding amplifying tube 99 through an interstage network as shown. Theplate voltage and the positive screen gridvoltage of the tubes H and 59 are fed by the same plate battery 28 through the usual connecting impedance elements from lead 40. The second amplifying tube is is connected to an output or power tube 24 through aninterstage network similar to that preceding the tube l9..

The output or power tube 2.4 is a 3i i-A Western Electric power amplifier pentode. The plate of .the power amplifying pentode is connected to the positive pole of the plate battery through lead 4! in series with an inductance 25 and through the customary primary winding of the output transformer. The cathode of the power.

tube is connected to the negative side of the plate supply battery and to ground through an 1 inductance 27.

In shunt therewith is a condenser 28 furnishing an alternating current bypass to ground.

The battery '29 employed in Fig. 1 for both filament lighting and plate voltage supply is a.

single storage ,battery of about volts subdivided bytaps into n, in this case, seven sections, 11, b, 0, etc. The amplifier A has its filaments lighted from sectiona. has its filaments lighted from section b of the battery and so on while the eighth amplifier The amplifier B at arepeater station has its filaments lighted from the first section a of the 'batterythe ninth from section b and so on.

Where the number of amplifiers at a station is less than the number of sections in which the plate battery is divided, or is not an even multiple of such number, individual filament lighting sections not heavily enough loaded maybe loaded with dummy loads, such asleak resistamplifier A over lead 34.

amplifiers are such that the filament lighting and filament voltage battery iii of the prior art repeater station. The C battery 32 connects to battery and plate battery are employed in series for the plate voltage of the power tube; 24 with the filament battery below ground potential. The connection through the inductive winding 21 (see Fig. 1) over lead 3'! to the negative side of.

the filament heating battery 3! provides a good direct current path to the negative pole of the filament battery. The condenser 28 (see Fig. 1) and lead 36 give a good alternating current path amplifier panel and of the building housing the apparatus. The customary ground lead wire (Fig. 1) in the amplifier set is connected by. lead 36 to ground potential which is thepotential of the positive side of the filament battery.

The circuit connection of Fig. 2 thus provides the cathode of tube 24 with a direct current connection through its associated inductance 21 to the negative pole of the filament battery 3i, 2 4 volts lower than ground instead of providing the usual connection to ground. By this connection it is possibleto obtain the full voltage of about 154' volts of the combined A and B batteries instead of the usual 130 volts of the B battery alone for supply of the plate voltage for the power tube.

through the filament battery 3! while permitting the use of this battery as partof the effective plate supply. When the amplifier is to be used in a new office the power supply is thatoi Fig. l and the coil 27 and condenser 28 would not be needed because of the direct connection of the cathode to ground.

Fig. 3 shows an embodiment of this invention where a repeater amplifier is modified to use the combined filament and plate batteries for plate voltage on all of the tubes of each amplifier A, B N. Each and every cathode is at the potential of the negative side of the "filament battery 3| below ground so that the normal ground connecting wirewithin the amplifier is really at a potential negative with respect to the potential of the building ground. If, as is the common practice, the transformer shielding were connected to the amplifier panel and this were grounded as usual the cross-talk between amplifiersoperated from the two batteries as a common source of supply would be'excessive. The cross-talk trouble canbe alleviated by insulating the transformer shielding from the panel and connecting it to the common cathode return wire. This expedient has the disadvantage that a potential difference equal to the filament battery voltage, 24 volts in this case, exists between the transformer shielding and the closely adja- ;cent grounded panel. Any accidental short cir- The connections to the.

There results a material increase in the available power output from'the repeater cuit to ground would burn out fuses and disable repeater circuits. This invention makes it possible to obtain the electrical advantages sought without incurring the disadvantages. The shields for the input and output transformers are insulated from their supports and ground and are connected, back to a common cathode return wire 38 withinthe amplifier each through a condenser C1. Each shield is connected to ground itself The resistance R1 is shield at ground potential as far as direct current is concerned, yet large compared to the reactance of 01. These resistance connections between actual ground and any shielding, that may be necessary, protect the fuses while the condensers give thecross-talk advantage of direct connection to lead 36. Alternatively, the entire panel might be insulated from the relay rack and the entire amplifier treated in the ordinary manner, namely, by the connection of the low potential parts back to panel ground and by connecting vformerfonly results in an increase in cross-talk between the amplifier so grounded and'the other amplifiers but does not result in a short-circuit or in the blowing of a fuse.

The condensers 5!), 5| and 53 illustrated as connecting parts of the transformers to outer shields are virtual or inherent condensers only, indicating the naturally existing capacity between the parts of the transformer and the shielding means. It is these virtual or distributed capacities that cause undesired cross-talk'if any shield or exterior "shielding became accidentally grounded on a supporting frame.

The chokecoil connection 21 noted in Fig. 1

from the cathode to actual ground is omitted in the amplifier of Fig. 3 since it is unnecessary to connect the cathode of the power pentode to ground through a choke, the ground being identical for all of the tubes in the amplifier of Fig. 3.

The power supply systems disclosed are not limited in application to telephone repeater service or to the use of either wet or dry battery supply The subject-matter of this specification directed to the embodiment exemplified by Fig. 1 is; also being disclosed and is being specifically claimed in the copending application Serial No. 228,987 of I. G. Wilson, filed September 8, 1938.

What is claimedis:

1. The combination of a plurality of amplifiers, a source of cathode-heating current common to said amplifiers, said source being connected to said amplifiers such that cross-talk between said amplifiers may take place through said souce, and means including an inductive impedance connected between said source and each amplifier for limiting the flow through said source of alternating current from said amplifiers.

2. In combination, a plurality of amplifiers, each including a stage having an electron discharge device with a cathode, a cathode-heating filament and an anode, a source of heating current for said filament, said source being common to all of the amplifiers and providing a portion of the eifective anode potential for said stage, and

means for reducing cross-talk between said amplifiers through said common source, said means including an inductive impedance connected between each cathode and the low potential terminal of said source.

3. In a telephone repeater station, the combination of a plurality of amplifiers, each of which is in a separate communications circuit and each of which includes a stage having an electron discharge device with a cathode and an anode, a common source of current for heating the cathodes of said devices,each of the cathodes being connected to the low potential terminal of said source, the high potential terminal being connected to ground, a common source of anode potential having its low potential terminal connected to ground and its high potential terminal connected to the anodes of said devices, and means for reducing the flow through said common heating sourcev of the alternating current component of the cathode-anode circuit current of each amplifier, whereby cross-talk between said amplifiers is minimized.

4. The combination of the next preceding claim in which said means comprises a high alternating current impedance path between each cathode and the heating source and a low alternating current impedance path between each cathode and ground.

JAMES O. EDSON. IRA G. WILSON. 

